The silicon nitride sintered material, that is a material excellent in thermal characteristics and in mechanical characteristics, has been described in various Patent Documents and technical literature.
Especially noteworthy is a silicon nitride sintered material having silicides of Fe, W, Cr, Mo or the like caused to precipitate in the grain boundary layers thereof for the purpose of improving the mechanical properties (refer to Patent Documents 1 through 4).
For example, Patent Document 2 describes a silicon nitride sintered material that has crystal grains of silicide of at least one of W, Mo, Cu, Mn, Fe and Nb dispersed in the grain boundary layers of the sintered material. Patent Document 3 describes a silicon nitride sintered material that has compound of metal having a high melting point, Fe—Si—O, formed in the grain boundary layers thereof. Patent Document 4 describes a silicon nitride sintered material that contains grains formed from silicides of W, Fe or the like and Ti compound (nitride, carbonitride or carbonate nitride) in the grain boundary layers thereof, while the silicides of W, Fe or the like coagulate around the Ti compound.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 5-148031    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2001-206774    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2001-106576    Patent Document 4: Japanese Unexamined Patent Publication (Kokai) No. 11-267538